In the absence of automatic control devices, the production of the faulty fabric goes on until the fault is visually noticed by the machine operators. This happens with a delay depending accidentally upon various factors, since the operator usually attends to various machines and to various tasks on each machine, beside controlling the product quality.
In the endeavour to overcome this drawback, there have been provided devices for the continuous and automatic check of the needles during the operation of a knitting machine. Most of the devices which are being presently marketed are based on electro-optical techniques which exploit the principle of the optical reflection.
Devices of this kind are described for example in U.S. Pat. Nos. 4,027,982 and 3,937,038.
Such reflection-type devices comprise a light emitting device, able to direct a light beam in the direction of a detecting area which is crossed in sequence by the needles during the operation of the machine, and a receiving device, which receives the light reflected by the needles. If the needles are not correctly positioned, are deformed or broken, the quantity of reflected light energy changes, and this is detected, at least theoretically, by the receiving device.
The electro-optical devices based on the principle of the optical reflection, however, have a number of drawbacks. First of all, installing the detector is difficult because of the need to search critical alignment conditions with respect to the path of the needles, conditions which may be kept with difficulty.
Furthermore, the device is not precise, since it tends to overview broken needles or to consider unbroken needles as being broken. Consequently, wrong decisions are taken in controlling the operation of the machine.
Finally, the performance of the device depends in an unacceptable way from the cleanness condition of the detector.
Solutions have also been tested based on the principle of the optical transmission, as, for example, the devices disclosed in U.S. Pat. Nos. 3,659,346 and 3,946,578.
For instance, the device disclosed in U.S. Pat. No. 3,946,578 comprises a light emitting device, able to direct a light beam in the direction of a detection area crossed by the needles, and two (or more) receiving devices (or surfaces) which receive the light emitted by the emitting device.
The needles, travelling across the detection area, intercept part of the light directed towards the receiving devices. In this manner, such receiving devices are able to show when the respective portions of the detection area are crossed by each needle.
The detection of the faulty needles is made by checking that the needles cross all the portions of the detection area controlled by the receiving devices, in the correct time sequence. The detection is possible in fact due to the specific shape and arrangement of the receiving devices, which are such that the faulty needles do not cause a correct time sequence.
This detection system has drawbacks similar to those of the reflection-type devices. As a matter of fact, for a correct operation of the device, it is necessary that the configuration and installation of the detecting electro-optical devices are very accurate and designed specifically for controlling a given type of needles.
In this way, the operation of the device is very critical, particularly with respect to variations of position of the installed device, vibrations, as well as variations in speed or type of operation of the knitting machine.
These drawbacks of the devices of the prior art render the performance of such devices not fully satisfactory, and cause a decrease of such performance during the operation.
GB-A-1 186 985 discloses a device for continuously monitoring the needles of a knitting machine during operation thereof, comprising:
a detecting optical head, comprising a main structure carrying: PA1 a light emitting source, PA1 first optical means for receiving the light emitted by said source and guiding said light up to an emitting window, for emitting a light beam obtained thereby in the direction of a detecting area which is crossed in sequence by the needles of the knitting machine during operation thereof, PA1 a receiving window which is located in such a way as to receive said light beam after that the latter has crossed the detecting area, PA1 second optical means for receiving the light of said beam coming into said receiving window and for guiding said light up to an electro-optical receiver, said receiver being able to emit an electric signal indicating the light energy which comes to the receiver as being not intercepted by the needle which is at each time at the detecting area, and PA1 a control electronic unit for receiving the signals emitted by said electro-optical receiver and processing such signals in order to detect any broken needle passing through the detecting area.
A device of this kind is generally able to select between broken and unbroken needles, but is not able to secure an information on the general configuration of the needle, so that unbroken needles which are nevertheless defective, such as bended needles, cannot be detected. Moreover, the control operation carried out by the electronic unit of this known device is based on a comparison between the output data relating to the needle which is being monitored and standard reference data, so that the monitoring operation may be affected by operating conditions of the machine.
The object of the present invention is to overcome all the drawbacks of the prior art.